The closing speed of a door closer is usually controlled by controlling the flow velocity of oil therein. However, the viscosity of oil varies upon changing temperature, to affect the closing speed of door closers greatly in the regions having big diurnal temperature amplitude, e.g. North America and the North China. It can be known from the experiments, the closing time of a hydraulic door closer is about 8 seconds at 25° C., but when the air temperature dropped down to minus 20° C., the closing time is increased to about 2 minutes, which is 15 times the former. A much longer closing time can cause many problems, for example, the cold air in winter can enter the house easily, or a security flaw will appear. Moreover, it is difficult for users to adjust the door closer, the above flaws should increase the maintenance service cost and may incur a risk of oil spilling.